Effect of framework design and firing cycle on the success rate of bilayered zirconia crowns

Objective: This study investigated the effect of framework design and modification of porcelain firing cycle on the occurrence of fractures of metal-ceramic and bilayered zirconia crowns after cyclic loading. Method and materials: Twenty-four zirconia crowns with two different apicocoronal incisal veneering porcelain lengths (ZS = 2.0 mm and ZL = 4.0 mm) were fabricated. Twenty-four metal-ceramic crowns with the same apicocoronal incisal veneering porcelain lengths (MS = 2.0 mm and ML = 4.0 mm) served as the control. All the specimens (n = 48) were thermally and mechanically cycle-loaded (49 N) for 1,200,000 cycles, and then evaluated for cracks and/or bulk fractures with an optical stereomicroscope (×10). In the second part of the study, the influence of firing cycle modification on the success rate of twenty-four bilayered zirconia crowns was also evaluated. Finally, two specimens from each group (n = 12) were sectioned and analyzed using a scanning electron microscope. Load-to-failure of the specimen that did not present bulk fracture was further assessed using a universal testing machine. Statistical analysis was performed with Fisher exact test and Kruskal-Wallis test. Results: A statistically significant lower occurrence of fractures (P .05) was found after cyclic loading for bilayered zirconia versus metal-ceramic crowns. Modification of the firing cycle did not significantly influence the outcome after cyclic loading for zirconia bilayered crowns (P > .05). Conclusion: Within the limitations of this study, porcelain-firing cycle with an extended cooling time did not appear to influence the occurrence of cracks and fractures of bilayered zirconia crowns. Keywords: all-ceramic, cyclic loading, dental technology, failure analysis, firing cycle, metal-ceramic, porcelain, prosthodontics, zirconia